This invention relates to an electrode, a method of fabricating the electrode, and a battery using the electrode. More particularly, it relates to an electrode whose resistivity changes with a rise in temperature, a method of fabricating the electrode, and a battery using the electrode.
In recent years, with the development of electronic equipment, batteries used therein as a power source have increasingly gained in capacity and output density. A lithium ion secondary battery is attracting attention as a battery fulfilling these requirements. A lithium ion secondary battery has an advantage of high energy density but requires sufficient measures for safety because of use of a nonaqueous electrolytic solution.
Conventional safety measures include a safety valve which relieves an increased inner pressure and a PTC element which increases resistivity on heat generation due to an external short-circuit to shut off the electric current. For example, incorporation of a safety valve and a PTC element into the cap of a positive electrode of a cylindrical battery is known as disclosed in JP-A-4-328278. However, on the safety valve""s working, moisture in the air enters the inside of the battery, which can induce an exothermic reaction in case lithium exists in the negative electrode.
On the other hand, a PTC element, which cuts off the external circuit involving a short-circuit, exerts no bad influence on operating. The PTC element can be designed to operate when the battery temperature rises to, for example, 90xc2x0 C. or higher due to an external short-circuit so as to be the first safety element to operate in case of abnormality.
Having the above-mentioned structure, conventional lithium secondary batteries involve the following problem. When a short-circuit occurs in the inside of the conventional lithium secondary battery to raise the temperature, the battery is incapable of suppressing an increase in short-circuit current.
In the case where a short-circuit occurs in the inside of the lithium secondary battery to raise the temperature, a separator made of polyethylene or polypropylene interposed between a positive electrode and a negative electrode is expected to soften or melt to clog the pores of the separator, whereby the separator would exude a nonaqueous electrolytic solution contained therein or would seal the nonaqueous electrolytic solution within itself to reduce its ion conductivity thereby to diminish the short-circuit current. However, the part of the separator distant from the heat generating short-circuit does not always melt. Besides, when the temperature rises, it is likely that the separator melts and flows to lose its function of electric insulation between positive and negative electrodes, which can lead to a short-circuit.
In particular, in the case of a lithium ion secondary battery, the negative electrode is prepared by coating a substrate functioning as a current collector, such as copper foil, with a slurry comprising a negative electrode active material such as graphite, a binder such as polyvinylidene fluoride (PVDF), and a solvent, and drying the coating layer to form a film. The positive electrode is similarly prepared in a film format on a substrate functioning as a current collector, such as aluminum foil.
The positive electrode comprises a positive electrode active material, such as LiCoO2, a binder, and a conducting agent. The conducting agent is to enhance electron conductivity of the positive electrode in case where the active material has poor electron conductivity. The conducting agent to be used includes carbon black (e.g., acetylene black) and graphite (e.g., KS-6).
When the temperature of such a battery increases to or above the temperature at which a separator between the positive and negative electrodes melts and flows due to, e.g., an internal short-circuit, a large short-circuit current flows between the positive and negative electrodes in the region where the separator flows. It follows that the battery temperature further increases by heat generation, which can result in a further increase of the short-circuit current.
The invention has been made in order to solve the above-described problem. An object of the invention is to provide an electrode which increases its resistivity with temperature, a method of fabricating the electrode, and a battery using the electrode.
A first electrode according to the invention comprises an active material, a conducting agent which is in electrical contact with the active material, and a resin which is in direct contact with the active material and/or the conducting agent and expands in volume with a rise in temperature. According to this aspect, the resin expands to increase its volume with a rise in temperature. As a result, the direct contact between the active material and the conducting agent is broken off to increase the resistivity of the first electrode. Accordingly, the current flowing through the electrode can be prevented from increasing in case of a temperature rise.
A second electrode according to the invention is characterized in that the resin has a melting point ranging from 90xc2x0 C. to 160xc2x0 C. Since a resin having a melting point ranging from 90xc2x0 to 160xc2x0 C. is used, the resistivity increases at a prescribed temperature or thereabouts within the range of from 90xc2x0 to 160xc2x0 C.
A third electrode according to the invention is characterized in that the resin has a particle size of from 0.05 xcexcm to 100 xcexcm. The particle size of the resin ranging from 0.05 to 100 xcexcm, the electrode increases its resistivity at a prescribed temperature or thereabouts, and a battery using the electrode has an increased discharge capacity.
A fourth electrode according to the invention is characterized in that the resin is a crystalline resin. Containing a crystalline resin, the electrode exhibits a further increased rate of change in resistivity at a prescribed temperature or thereabouts.
A first battery according to the invention has a positive electrode, a negative electrode, and an electrolytic solution provided between the positive and the negative electrodes and is characterized in that the positive or negative electrode is any one of the above-described first to third electrodes. According to this aspect, since any one of the first to third electrodes is used as the positive or negative electrode, the battery has improved safety. That is, in case where the inner temperature of the battery rises to or above a prescribed temperature, the electrode increases the resistivity thereby to reduce the current flowing inside the battery
A first method of fabricating an electrode according to the invention comprises the steps of:
(a) dispersing the above-described conducting agent, active material and resin powder to make an active material paste and
(b) pressing the active material paste having been dried at a prescribed temperature under a prescribed pressure.
A second method of fabricating an electrode according to the invention is characterized in that the prescribed temperature is the melting point of the resin or thereabouts. Since the prescribed temperature is the melting point of the resin or thereabouts, the resin undergoes deformation and enters among adjacent positive electrode active material particles, among adjacent conducting agent particles, and between the positive electrode active material and the conducting agent that are adjacent to each other. Therefore, the resin efficiently functions in severing the contact among the positive electrode active material particles, the contact among the conducting agent particles, and the contact between the positive electrode active material and the conducting agent in case of abnormality. As a result, the increase in electrode resistivity can be made higher in case of a rise over a prescribed temperature.